Police have launched a manhunt after a wealthy American socialite who was the first person in London to enter a civil partnership was found battered to death.

The body of Robert Troyan, 63, was discovered by his cleaner lying in a pool of blood at his flat in the wealthy London area of Mayfair. A post mortem confirmed he died as a result of head injuries.

Troyan, known as "Roberto", had lived alone in the flat since his partner Anthony Feldman, a renowned architect, interior designer and composer, died of pancreatic cancer in 2005.

The pair had been together since 1983 and entered into a civil partnership just one month before Feldman's death, becoming the first couple in the capital to do so.

A Times obituary for Feldman, who died aged 52, described how Feldman worked for Jonathan Aitken, the former MP, and Princess Michael of Kent, among others.

During his time with Feldman, the pair lived in the top two storeys of an 18th-century townhouse where they were said to be famed for their dinner parties. According to reports, Troyan had studied at art school in America before moving to the UK. He is believed to have collected antiques, which were stored at his flat.

One neighbour told the Evening Standard that Troyan was known as a "colourful character". He said: "He came from a very monied background in America. He was totally devastated by Anthony's death. It was Anthony that really kept everything running smoothly in both their lives.

"After Anthony died, there was quite a lot of coming and going of various characters, and it caused some problems. Nothing would surprise me. In the end Robert moved to Mount Street. I went past there on Friday, and I actually thought – 'I hope it's not him'."

Detective Chief Inspector Tim Duffield said: "We are appealing for anyone who knew Robert to come forward. He was a very well-known character in Mayfair, whom friends describe as kind and extremely generous."